1. Field Of The Invention
This invention relates to a device and method for measuring reflectivity of small irregular surfaces, and more particularly relates to measuring reflectivity of the surface coating on cathode ray tube heaters.
2. Prior Art
When current is applied to the heater in a cathode ray tube, heat is produced due to the resistance of the heater wire. The heat flows to the cathode and to the emissive material on the cathode causing electrons to emit and flow through the gun to the screen of the cathode ray tube. The amount of electrons emitted from the emissive material is a function of the amount of heat applied to it, so it is important to control heater characteristics.
The shade (also called reflectivity or emissivity) of the heater coating for cathode ray tubes is an important characteristic because heat is removed from the heater by radiation. Shade is controlled by a multicoating process. The undercoat is aluminum oxide which results in a white color coating. The overcoat is a darker coating of tiny tungsten particles. The more dark particles that cover the white coating, the darker the shade. Cathode temperature is a function of the amount of power supplied to the heater. Power is controlled by voltage and current. Since heater voltage is normally fixed at either 6.3 or 7.1 volts, any variation in heater power is due to variation in heater current. Heater current is controlled by three main factors; heater design, wire thickness and heater shade. Heater shade affects the current because the wire used in the heater is normally tungsten or a tungsten alloy and the resistance of the wire varies with temperature. Thus, with a darker shade of overcoat, energy is radiated away from the heater faster, the heater runs cooler, the resistance of the wire decreases, the current increases and the power of the heater increases. Thickness of the wire has a similar effect. When wire thickness increases, the resistance decreases, the current increases and the power increases.
Wire thickness varies from lot to lot, and it is difficult to obtain wire having thicknesses controlled within limits sufficient to maintain the heater power within acceptable limits. Thus, it is necessary to change the shade of heaters to compensate for the variation in heater wire thickness.
Means that are presently available to measure reflectivity of this coating include visual inspection using photo gray shade cards and an optical reflectance device, both of which are acceptable for measuring reflectivity of large areas but are unsatisfactory to measure reflectivity of cathode heaters because the heaters have small irregular surfaces. Use of either device results in poor repeatability of readings, poor accuracy and calibration difficulties. Also, the optical reflectance method prevents comparison of the reflectivity of different heater types because the readings include the background and all the coils. Thus, readings will vary as the size of the wire or the number or size of the coils vary with different heater types.
Accordingly, objectives of the present invention include: providing a device which will improve repeatability of readings, improve accuracy, reduce calibration difficulties and permit comparison of reflectivity measurements for different heater types.